The functions or properties required for coatings include adhesion to a material to be coated, protection of the coated material and good appearance. Various types of coatings have been developed for these purposes and are put in practical use. Off course, the factor that influences and defines the properties of a coating is a coating resin used in the coating.
The materials to be coated themselves have been bewilderedly changed with the progress of times, and recently more complicated properties and functions have been required for the materials, such as light weight, frame retardancy, recycling properties, easy biodegradability and safety, as well as high strength. Under these circumstances, as the materials to be coated, the following materials have been often used: metals having light weight, high strength and abundant resources and easy recycling properties (e.g., magnesium alloys, aluminum alloys and titanium alloys) as construction materials; plastics having high strength and good transparency (e.g., polycarbonate) as alternatives to glass; and plastics having good flame retardancy (e.g., Noryl, PPS and PC/ABS resin alloy) for application in household electrical appliances.
These materials are generally hard adhesive, and particularly those materials which contains as a constituent a metal atom (e.g., Mg, Al and Zn), such as magnesium alloys, aluminum alloys and titanium alloys, appear to be hardly adhered with a coating. From the standpoint of coatings, these materials are deemed to be hard to adhere, and widely available acrylic melamine resin coatings, acrylic urethane resin coatings, acrylic silicone resin coatings and the like cannot adhere to the surface of these materials. Therefore, use of primers such as so-called “two-part” epoxy resin coatings are needed.
Inconveniently, these metals belong to the most non-noble class from the viewpoint of electroconductivity. Therefore, there are frequent troubles such as electric corrosion on the coated material due to improper application of the coating to the metals, blistering of the coati film observed in the environmental tests (e.g., water resistance test), or readily peeling of the coat film.
To improve and ensure the adhesion of a coating to the materials, it has been widely employed to apply an epoxy resin coating, which is known to have good adhesive properties, as an under coat (primer). Subsequently, the epoxy resin coating is baked, cured, and coated with a top coating (e.g., acrylic melamine resin coating, polyester resin coating, fluorine resin coating), baked and cured. However, in this treatment, it is difficult to ensure the sufficient corrosion resistance of the base material. In practical applications, chemical conversion treatments has been employed, such as formation of a chromium chromate film or a phosphate-chromate film.
When a chemical conversion film is formed as mentioned above, this coating process can provide a coat film having good properties by virtue of the outstanding adhesion and corrosion resistance of an epoxy resin. However, the coating process is called “2C2B” (two times of coating and two times of baking) or “3C3B” (three times of coating and three times of baking; often performed when the coating appearance is disapproval), and the coating workability is quite poor and the yield of the coating is low. Moreover, it takes much time for the coating pretreatment (chemical pre-treatments, such as chemical conversion treatment with a chromate, etc.) and the coating process, the coating cost increases, and the efficiency of production of final products is extremely low. Since the epoxy resin forms a tightly cross-linked coat film, the adhesion of a re-coating is insufficient.
The present invention has been made for overcoming the above-mentioned problems. Accordingly, the object of the present invention is to provide a coating having an excellent balance of various properties and a good adhesion to hard adhesive materials, in particular hard adhesive metals and plastics such as magnesium alloys, aluminum alloys, PPS and polypropylene, by using a specific acrylic resin as a coating binder.
Another object of the present invention is to remarkably improve the corrosion resistance of an electrically less noble metals such as magnesium alloy and aluminum alloy.
Still another object of the present invention is to provide a coated article which is coated with the coating, in particular coated article containing, as a constituent element, a magnesium alloy, an aluminum alloy, a titanium alloy, PPS, polypropylene, ABS resin, PC/ABS resin alloy and the like.